Measurement of attenuation and change of refractive index due to water vapour in the atmosphere at 890 GHz

An open-resonator technique is used to measure the attenuation and change of refractive index at 890 GHz for a variable water-vapour content in the atmosphere, using an HCN laser as a source. The refractive index increases uniformly with water-vapour content, a change of relative humidity from 0 to 50% at 26°C resulting in an index change of 1.3 × 10¿4. An attenuation of 85±20 dB/km was measured for 50% relative humidity at 26°C.